Since one-year-old to be 0.07% and from CT

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Since the advent of clinical computerized technology
(CT) in the early 1970s by Sir Godfrey Hounsfield its use in the investigation
of both adult and pediatric pathology has continued to grow1. The
last three decades have seen major advances in CT. Over the recent years there
has been increasing concern about the long term effects of exposure to ionizing
radiation particularly in the pediatric population. Children’s less mature,
rapidly dividing tissues are more sensitive to the effects of ionizing
radiation. In addition, children’s longer life expectancy means they have a
much longer latent period of oncogenic effects of ionizing radiation compared
with adults. Several studies have tried to estimate the risk of radiation
induced cancer from pediatric CT2. They estimated the lifetime
cancer mortality risks attributable to radiation exposure from CT head in a
one-year-old to be 0.07% and from CT abdomen to be 0.18%. More recent data
suggest that the brain is significantly more radiosensitive than was previously
thought, the risk increasing with decreasing age.

The estimated risk of cancer death for those undergoing
CT is 12.5/10000 population for each pass of the CT scan through the abdomen3.Therefore,
concerns regarding a reduction in radiation dose have been recently raised
during CT acquisitions4. Although decreasing tube current is the
most common means of reducing CT radiation dose5-8, this alteration
also reduces the contrast to noise ratio (CNR) which may affect the diagnostic
outcome of the examination.

Although CT comprises a relatively small fraction (4%)
of all radiological examinations, it contributes to as much as 35% of the
collective radiation dose to the population from radiological examinations,
since the radiation dose for each CT examination is relatively high9.
Several authors have focused on reducing the radiation dose to the patient by decreasing
the mAs-value10-12. Doing this increases image noise, and if the
diagnostic accuracy is still acceptable, it is a profitable way of reducing the
radiation dose.

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Most centers use 120 kVp, but there is no consensus
over optimal tube current13. Tube currents from 200 to 533 mAs for
chest CT have been reported. Tube currents have been chosen arbitrarily without
assessing impact on image quality and lesion detectability. Appropriate tube
current is more difficult to define for CT than for conventional radiography
because CT is a digital technique in which acquisition and display are not
related. Therefore when tube current is excessive, the CT image does not become
too dark but merely improves because of decreased image noise. Because
radiation dose is linearly related to amperage at a fixed kilovoltage,
reduction in the miliamperage or tube current used is equivalent to dose
reduction. Thus optimal CT tube current is an appropriate balance between image
quality and radiation14.

Image quality in CT, as in all medical imaging,
depends on 4 basic factors: image contrast, spatial resolution, image noise,
and artifacts16. Depending on the diagnostic task, these factors
interact to determine sensitivity (the ability to perceive low-contrast
structures) and the visibility of detail. In radiography, image noise is
related to the numbers of X-ray photons contributing to each small area of the
image (e.g., to each pixel of a direct digital radiograph)15.

In the brain, use of CT has to be balanced against the
need to minimize radiation exposure and the increased availability and use of
MRI to manage intracranial pathology in some settings1. MRI gives
superior differentiation of the tissues but the long scan times inevitably
means that patient motion becomes an issue resulting in increased need for
sedation or GA. 

Categories: Data


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